Compression molding for aerospace composites

From design to manufacture, it can be challenging to achieve product consistency with aerospace composite materials. Manual processes can lead to expensive, time consuming rework machining for finished products, especially with complex, layered materials such as carbon fiber reinforced plastics (CFRP).

However, using the correct composite production technology will reduce the variation within a single component as well as from one component to the next.

“Good process control provided by new compression molding machines improve the quality of the product and allow manufacturers to gain a deeper understanding of the process,” says Mike Josefiak, mechanical engineer with Greenerd Press & Machine Co.

Josefiak recently sat down with Aerospace Manufacturing and Design editors to discuss how compression molding can improve parts consistency as aerospace companies increasingly turn to lightweight, difficult-to-process composite materials.

AM&D: What is compression molding, and how does it differ from other hydraulic forming technologies?

MJ: Compression molding uses temperature and pressure, over time, to form, cure, and/or bond a component. The material may be a fabric impregnated with thermosetting resins, a partially cured rubber, a molding compound – bulk molding compound (BMC) or sheet molding compound (SMC), or other materials that benefit from a heating cycle under pressure.

AM&D: What are the best applications for compression molding?

MJ: The desire for higher strength-to-weight ratios in aerospace applications makes it a natural fit for composite materials. Compression molding creates structural components for modern aircraft interiors, replacing aluminum for weight and cost savings.

In more demanding applications, such as the CFM Int’l. LEAP engines powering the A320neo and 737 MAX, composite fan blades are made in a compression molding process. The high bypass ratios being used to reduce fuel consumption have increased blade size. These longer blades must resist bird strikes with flexibility and strength, always keeping component weight to a minimum. This requires tight process control and repeatability in the molding process.

Compression molding functions well, producing components that have less complex or fine features because many materials do not flow freely. The best applications will be thin – reducing the time needed to cure material at the core of the component, so cycle times do not become unacceptably long.

AM&D: How does the process work?

MJ: Compression molding presses are very flexible and can be tailored to meet the needs of a specific component or families of components. The three keys to a successful compression molding process are managing time, temperature, and pressure. Adjusting these factors precisely to meet the chemical and physical characteristics of a product ensure consistent, successful results.

Sizes can range from a few inches to many feet wide. Size, combined with the desired pressure on the material, will determine the machine’s capacity.

Pressures on the material range from less than 100psi to more than 3,000psi. Minimizing deflection of the machine platens, and controlling parallelism will keep pressure consistent across the working area.

Modern analysis and temperature control methods can keep temperatures within a few degrees. Systems compen- sate for heat absorbed by the working material and ambient condition changes to maintain consistent ramp rates and soak temperatures.

AM&D: Does the process work with thermoplastics as well as thermosets?

MJ: Yes, thermoplastics and thermoset materials can be used in this process, although low viscosity fluids can be difficult to contain within a metallic die, and flashing is to be expected.

AM&D: Can compression molding be used on metal parts?

MJ: The same style of presses used for compression molding can also be used in a warm- or hot-forming application for metallic parts. While the energy and forces required for these operations generally increase, the same principles of controlled heat and pressure can improve production of metallic parts susceptible to tearing or thinning. Heating many materials to a small fraction of their melting points makes it easier for materials to flow, helping reduce the scrap rate of demanding forming work.

AM&D: How do traditional hydraulic presses and compression molding presses differ?

MJ: Compression molding presses typically use hydraulic power systems because of the low power demand required for holding a product under pressure, as well as being a cost-effective way to achieve tight force control.

Hydraulic press systems designed specifically for compression molding will take additional steps to smoothly transition pressure on the product, maintain equal force across the work area, and minimize energy consumption.

AM&D: If starting a compression molding operation, what features should be considered in a press?

MJ: Control systems that manage temperature, time, and pressure independently throughout the cycle will give end users the tools they need to succeed. Every product is different, so the ability to test and adjust conditions consistently leads to reduced cycle times and lower scrap rates.

AM&D: Are there standard press features, or are they all customized to part-making requirements?

MJ: Temperature control of the working surface, force control, and the ability to hold pressure for long periods of time are all basic functions in compression molding presses. However, there are several options to consider. The materials being formed and the particularities of each die will play the largest role in which options can benefit a manufacturer.

Heater-zone controls can ramp up temperature at a controlled rate across the full working area. Additional cooling systems provide similar controlled temperature ramp-down. The level of control and speed of temperature change can be configured to match the needs of the materials.

Position control can be added for products with a required thickness, holding positions as close as ±0.001". On large products, this may be multiple cylinders working in concert to maintain the position, even as the material being formed pushes outward on the dies.

Vacuum chambers around the working area help remove additional gas within the product, as well as limit oxidation or contamination problems.

These are only a few of the possibilities. Greenerd Press & Machine compression molding presses are readily expandable, supporting any number of operating conditions to speed up production, reduce scrap rates, and improve the end product.

AM&D: What operator training is needed, and how is it provided?

MJ: Operators can be trained on-site in basic operation in less than an hour. Basic systems typically use time, temperature, and pressure setpoints for adjustment.

Advanced presses can become more complex, using step programming with ramp rates for temperature, position, and force throughout set time periods. To get the maximum potential from these systems, it is critical to gain a deep understanding of the material being formed and how the conditions around that material benefit the process.

Greenerd Press & Machine Co. www.greenerd.com
IMTS 2018 Booth #236402